There are many processes and techniques today for fluid-assisted (gas or liquid) injection molding. Fluid-assisted injection molding processes have added flexibility to the design and manufacture of plastic parts with their ability to produce partially hollow, lightweight, rigid parts with minimal sink marks and less tendency to warp. These processes can reduce material requirements, as well as equipment costs and cycle time, and thus have advantages over conventional injection molding processes and techniques in many applications.
In general, fluid-assisted injection molding systems utilize another material, either in a liquid or gaseous state, under pressure to expand the plastic material in the mold and conform it to the mold cavity details. The liquid or gaseous fluid can be introduced into the mold in several ways, such as through a bushing, nozzle, or machine nozzle in one or more cavities, or in more than one location. In conventional plastic injection molding processes, the molten plastic material is injected through sprue bushings or nozzles (heated or unheated) into the cavities of hollow molds. Often, two or more sprue bushings attached to a common manifold are used. In this instance, the plastic melt coming from the injection molding machine through runners is distributed by a heated distributor block (a/k/a manifold) to the individual sprue bushings. One conventional method used to control the melt flow utilizes one or more pin or needle valves, also known as valve-gate bushings.
With these systems, a needle valve is inserted through the manifold into the sprue bushing and controlled for axial movement by a hydraulic, electric or pneumatic control device or mechanism. The needle valve has an elongated pin which is moved axially by the control mechanism and is adapted to fit within an orifice in the end of the sprue bushing in order to open and close the passageway of plastic melt from the sprue bushing into the mold cavity. One of these systems is shown, for example, in U.S. Pat. No. 4,279,582.
Another type of plastic injection molding method and system utilizes a co-injection apparatus. This apparatus injects two different plastic materials, typically an inner core material and an outer shell material, into a single mold cavity. A co-injection manifold receives material from two different injection machines and combines the two materials into a single stream that flows into a mold or die.
Co-injection processes create a product which is less expensive. A smaller amount of the more expensive outer plastic material is used for the final part, since it only is used for the outer surface or shell of the product. The second material which is co-extruded is typically a less expensive plastic material and forms the inner non-visible core of the product. A co-injection manifold and process are shown, for example, in U.S. Pat. No. 4,376,625.